292 PRINCIPLES OF ANIMAL BIOLOGY 



accidental dispersal across barriers or dissemination by man has taken 

 place. 



Difference in Physical Conditions in Different Ranges. — The range 

 of certain animals is in a general way coincident with areas characterized 

 by certain general physical conditions. In North America, the open, 

 treeless areas of the west comprise the principal range of the prong-horn 

 antelope, bison, ground squirrels, and many other forms. The eastern 

 deciduous forest areas are inhabited by many forms that are mostly 

 restricted to them, as the opossum, grey fox, fox squirrel, cardinal bird, 

 Carolina wren, and yellow-breasted chat. The coniferous forests of the 

 north are the home of many species which extend little if any beyond 

 them, such as the snow-shoe rabbit, pine martin, northern jumping mouse, 

 three-toed woodpecker, and spruce grouse. 



The fact that within areas characterized by certain physical conditions 

 are found animals and plants which do not extend far beyond their limits 

 has led to the recognition of major environments (Fig. 203), which may 

 be divided into smaller environments (Fig. 204) . The conspicuous plant 

 associations of these environments afford a convenient means of designa- 

 tion, and coniferous forest areas, deciduous forest areas, bunch-grass areas, 

 etc., are recognized. The environments are not sharply separated but 

 intergrade on their common boundaries. Often this intergradation takes 

 the form of interdigitation; that is, the regions dove-tail together. The 

 .interdigitation of the plains areas of the United States with the eastern 

 deciduous forest regions is described by Ruthven as follows: 



"It has been shown elsewhere that the prairie region cannot be merged with 

 the forest regions of the states east of Illinois, but tiiat neither, on the other 

 hand, can it be classed with the arid plains. The environic conditions differ 

 from those of the eastern forest region in being more arid and principally char- 

 acterized by grass associations, and from the arid plains in being less arid and 

 supporting a greater tree growth. Furthermore and in harmony with these con- 

 ditions the terrestrial vertebrate fauna consists almost entirely of a mixture of 

 eastern and western forms. In view of these conditions the region must be con- 

 sidered as a transition area. 



"This point will not be discussed further, but it should be noted that the dif- 

 ferent elements in the fauna of northwestern Iowa occupy, as a rule, different 

 habitats — the prairie forms being western and the marginal forest forms eastern 

 types, while in general it is those associated with the aquatic conditions that occur 

 in both regions. For instance, it is in the marginal forests along the shores of 

 the streams and lakes (also now in the groves) that one finds the blue jay, Balti- 

 more oriole, red-headed woodpecker and many other forms characteristic of the 

 forests of eastern North America; it is on the prairie that one finds the prairie 

 hare, thirteen-lined spermophile, Franklin spermophile, western meadow lark, 

 grasshopper sparrow, prairie chicken, burrowing owl and many other distinctly 

 arid forms; while it is in the aquatic habitats that one finds the generally widely 

 distributed water birds and amphibious mammals." 



